Every 90 days (every 60 with pets). A clogged filter raises your energy bill and wears out the blower motor. The size is printed on the frame of the old filter.
Check monthly; most tanks need salt every 4–8 weeks. Keep the tank at least half full. If the salt has bridged into a crust, break it up with a broom handle.
Every 6 months. The indicator light lies in both directions — go by the calendar. Order the replacement when this nudge arrives; swap it when it ships.
Once a year — lint-clogged dryer vents are a leading cause of house fires. If clothes take two cycles to dry, do it now. A vent brush kit costs less than one service call.
Set the due time for the evening before pickup, recurring weekly. The classic squirrel: you know pickup day perfectly well — at 9am, in a meeting, watching the truck drive away.